urease activity in maize (zea maize l. cv. 704) as affected by nickel and nitrogen sources

nickel (ni) is one of the essential micronutrients for higher plants and its known function is beingthe metal component of urease. the effects of various ni levels on urease activity in maize (zea maize l.)plants grown in two nutrient media containing urea or ammonium nitrate as two separate nitrogen sourceswere investigated. the experiments were performed as completely randomized blocks with three replications.treatments included two growth media, the nitrogen of which was either urea or ammonium nitrate added atthe rate of 84 mg l-1 and four ni levels (0, 0.01, 0.05 and 0.1 mg l-1) supplied as niso4. plants were grownin the nutrient solutions for six weeks. on the second, fourth and sixth week of the growth period, both theleaves and root samples were taken to determine their urease activities. at the end of the sixth week, the dryweights of both the shoots and roots were also measured. urease activity in leaves of corn supplied with ureaincreased significantly with the increase in ni supply till the end of the 6th week sampling date, however inthose supplied with ammonium nitrate, urease activity increased up to the 3rd ni level and 4th week ofsampling date, but was reduced at the 4th ni level in the 6th week. urease activity in the roots of corn plantssupplied with urea was the highest at the 2nd ni level at the end of the 2nd week. increase in ni levels and date of sampling resulted in a decrease in urease activity. however, in ammonium nitrate-fed plants urease activity in the 2nd week of the sampling date increased up to the 4th ni level and for other sampling dates the activity increased up to 2nd ni level. further increase in ni supply and date of sampling resulted in a decrease in urease activity. enzyme activity was higher in the roots than in the shoots and was also higher in plants supplied with urea, compared to those fed on ammonium nitrate. in maize plants supplied with urea, the dry weights of the shoots and those of the roots were also higher.